1. Field of Invention
This invention relates to a method of treating lignocellulosic material such as wood pulp with chlorine for the purpose of removing the lignin remaining after the cooking process.
The bleaching process usually begins with a chlorination step, in which the lignin is converted into a water- or alkali-soluble form, whereby it can easily be removed. Because the chlorine is an inexpensive chemical compared with other bleaching chemicals, one strives to remove as much lignin from the pulp with it as is possible taking into consideration the quality of the pulp. This reduces the amount of chemicals required in the subsequent bleaching steps.
2. Prior Art
Until now chlorination of the pulp has usually been performed at a consistency of 3-4% in a chlorination tower, where the chlorine which has been dissolved in water is mixed into the pulp and allowed to react for a period of one hour. Taking into consideration the strength of the pulp an addition of chlorine dioxide is advantageous.
Chlorination of the pulp can also be performed as a gas chlorination, whereby the pulp from which the water between the fibres has been removed so that it has a consistency of about 30% is treated in a gas flow. The reaction time is then very short.
The delignification is dependent on the chlorine charge up to a certain limit value, which is reached when the amount of chlorine is about 1.3 g/g of lignin. It is possible to dissolve about 80% of the lignin in the pulp by chlorination in one step. If the pulp is washed, whereby the chlorinated lignin is removed and a new chlorination is performed, about 50% of the residual lignin can be dissolved i.e. about 90% of the lignin remaining in the pulp after the cooking process can be removed.
The methods which have been used until now require two chlorination steps and one washing step and cause furthermore a high consumption of chlorine if a high degree of delignification by the treatment with chlorine is desired.